This invention relates to a fluoropolymeric composite material. More particularly, this invention relates to a fluoropolymeric composite material which is particularly well suited for use as a bonding ply in a multilayer circuit board and in other electronic circuit applications requiring the ability to flow as well as good thermal, mechanical and electrical properties. The fluoropolymeric composite material of this invention also exhibits resistance to chemical degradation, particularly high pH (alkaline) environments.
U.S. Pat. No. 4,849,284, which is assigned to the assignee hereof and incorporated herein by reference, describes a ceramic filled fluoropolymer-based electrical substrate material sold by Rogers Corporation under the trademark RO-2800. This electrical substrate material preferably comprises polytetrafluoroethylene filled with silica along with a small amount of microfiberglass. In an important feature of this material, the ceramic filler (silica) is coated with a silane coating material which renders the surface of the ceramic hydrophobic and provides improved tensile strength, peel strength and dimensional stability. The composite material of U.S. Pat. No. 4,849,284 discloses a volume % filler fraction (on a void free basis) of at least 50 for use as a circuit substrate or a bonding ply.
The ceramic filled fluoropolymer-based electrical substrate material of U.S. Pat. No. 4,849,284 is well suited for forming rigid printed wiring board substrate materials and exhibits improved electrical performance over other printed wiring board materials. Also, the low coefficients of thermal expansion and compliant nature of this electrical substrate material results in improved surface mount reliability and plated through-hole reliability. As is known, individual sheets of this electrical substrate material may be stacked to form a multilayer circuit board. In fact, thin film formulations of the material disclosed in U.S. Pat. No. 4,849,284 (and sold by Rogers Corporation under the trademark RO-2810) may be used as a bonding ply to bond together a plurality of stacked substrate layers so as to form the multilayer circuit board.
It will be appreciated that high volume fractions (greater than 55 volume %) of ceramic filler will significantly and adversely effect the rheology (e.g. flow) of the ceramic filled fluoropolymer composite. This is particularly important when the composite is used as a bonding film or in filling openings in previously rigid structures. While ceramic filler volume fractions of 50-55% provide significantly improved rheological properties relative to higher filler fractions, there is a perceived need to provide even better flow properties to the fluoropolymeric composite without appreciably altering the excellent thermal, mechanical and electrical properties.
In addition to the adverse effects on rheology, it has also been determined that ceramic filler volume fractions of 50% and greater also have an adverse impact on the chemical resistance of the filled fluoropolymeric composite with respect to certain hostile environments, particularly high pH (alkaline) baths. Such high pH baths (e.g., greater than pH=12) are required for electroless copper deposition, which is a highly desirable and often used process in the manufacture of fine line circuitry (i.e., multichip modules). It has been found that high levels (50 volume % and greater) of ceramic filler may impart poor chemical resistance to the fluoropolymeric circuit material during long exposures in such alkaline baths. The resultant degradation caused by the poor resistance to the alkaline baths leads to a reduction in the hydrophobicity of the fluoropolymeric composite. In turn, this reduction leads to increased moisture absorption with a corresponding and highly undesirable decrease in the electrical properties of the composite circuit material.